Electron discharge apparatus



March 7, 1961 R. JEPSEN 2,974,253

ELECTRON DISCHARGE APPARATUS Original Filed Oct. 5, 1953 2 Sheets-Sheet1 I 21 23 I'll |5 4; 4 1 8 a it 225 '11 8'" E DISTANCE DISTANCE FI|5 :2-A :EI I5 & A

NVENTOR. EaaERr L. E'PsE ATTORNEY March 7, 196 R. L. JEPSEN 2,974,253

ELECTRON DISCHARGE APPARATUS Original Filed Oct. 5, 1953 2 Sheets-Sheet2 44 7 4 VII'IIIII/IA 43 Q as ea T6? 6/ o/srAucE- INVENTOR Ease-e r- L(/EPSEN FIE 2A ATTORNEV United States Patent fornia Original pp a 1 3,;Set-No: 3.84; .8 ew P te a a w. da d Max 9- Divi ed and this applicationApr. 13, 1952, Ser. No. 806,115

1 Claim. (Cl. 315-519)" This invention relates, generally, toultr a'high frequency electron tube devices or apparatus and the invention hasreference, more particularly, to noveljapparatus of this type whichremoves ions from the beam paths through the devices, such as klystrons,traveling wave and, other electron beam tubes, to thereby substantiallyeliminate perturbations of the beams thereof, and also to novel velocitymodulation tubes of the reflex klystron type which are so constructed'as to prevent reflected electrons from returning to the cathode region.

Several difficulties encountered in electron discharge devices such as,for example, klystrons and traveling wave tubes heretofore employedarise from an interaction be tween the electron beam and; ions trappedalong. the path of the beam. These difiiculti'es may include:

(1 Modulation of output power, frequency, and beam currentat thefrequency or frequencies ofcontinuous ion oscillation. These frequenciesare, typically, in the range .1 megacycle per second'to 1 0 megaeyclesper second.

(2) Modulation of outputpower; frequency andbeam current at both I a);Ion oscillation frequencies (in the megacycles per second range) and v(b) Low frequencies (typically in theaudio frequency range). The lowfrequency fluctuations are associated with the high frequencyio-no'scillations. (3) Changes in output power, frequency, and; beamcurrent as the frequency of an applied modulating volt-age is varied;These changes occur when the modulating frequency is near one of thefrequencies of incipient ionoscillation and they may be abruptorfluctuating. In many'heretofore existing reflex klystrons aqsourceoftrouble encountered is theoccurrence of what has been termedmultipletransitsof the beam. Multipletransits refer to the action ofthe'electr'on 'be'am after passing through; the resonator gap subsequentto being reflected back by thenegatively charged reflector. After thisre'- flecied or return trip through th e resonator gap, the bunchedelectron's haveperformed their useful function and t heir; eliminationfrom further operation onthe tube is desirable. However, manyofthereturning electrons enter the cathoderegion of the reflex klyst-lfonandare turned about by the negative charge tobeginj va'se cond 2 theklystron of the electrons in eliminated. 1 Another object of theinvention is to provide a novel reflex klyst'ron wherein ion trapping inthe re-ent'rant tube is substantially eliminated.

Another object of this invention is to provide a. novel method and;means for draining ions out from fined by grided apertures.

Still another. object of the invention is to provide a novel methodan'dmeans for draining ions from the rose; nator gap space in anelectron discharge device such as, for example, a reflex klys'tron.

the'beam are substantially These and other objects. and advantages ofthe present I invention will be better understood from perusal of thefollowing description wherein:

Fig. 1V is a schematic elevation view in section of a reflex klystronwhich-embodies the present invention whereby multiple transits of. theelectrons in the tube are prevented. and whereby. the ions arecontinually drained from the re-entrant tube or drift space between thecathode and the first resonator grid to prevent the forma- Fig. 1whereby the ions in the resonator gap space area three-cavityvelocitymodulation tube which embodies the novel invention for preventing theformation of ion I i roundtrip through the resonator gap, These multipletransits of the electrons produce undesirable effects in the tube suchas, for example, unstable power output.

Oneobject of the presentinyention is to provide a novel method and meansin electron beam. discharge devices for preventing the trapping of ionsin the beam path.

Another object of the present inventionis to provide a novel method andmeans for eliminating iontrapping in the beam. path of electrondischarge devices byinsuring that the'electron beam occupiesajprogressively increasing proportion of the space within, the,confining walls of the beampassage way as the beam sage way. I p

Another object of the present invention is to provide a progressesalongthe pasnpvel reflex klystron wherein mpltipleytransitslthroughtionofan ion trap therein.

' Fig. 2 is a section view'of the reflex klystron of Fig. 1 taken alongsection line 22.

Fig. 3' is a sectional view of a portion of a reflex klystron of thetype heretofore employed wherein. ions are trapped int'he drift space orre-entrant tube portion of thetube between the cathode and the firstresonator grid;-

Fig. 3(a) is a plot of the potential alongthe beam axis versus thedistance alongthe re-entrant tube space for the; reflex klystron shownin Fig. 3 showing the formation of an ion trap.

Fig.4 is a section view of. a portion of a reflex klystron whichembodies the present invention wherein the'walls of the re-e'ntrant tubespace between the cathode andthe first resonator grid are so formed asto prevent the forma-- tion of ion traps therein as occurs in theklystron of Fig; 3. This novel feature. is utilized in theklystron ofFig. 1". Fig. 4(a) is a plot of the potential along the beam axis versusthe distance along the beam path in the re-entrant tube space of Fig. 4showing. how the formation of ion traps is prevented.

Fig. 5 is a section view of. a portion of a reflex kly--' stronsubstantially identical to that shown in Fig. 1 which embodiesla noveladditional feature to the klystron'of also continually. drainedtherefrom to prevent the forma tion of anion trap. between the grids.

Fig. 6 discloses in section view a two-cavity klystron employing anannular shaped. beam of electrons wherein a novel method and means isprovided for continually draining ions from the re-entrant tube or driftspace between the accelerating grid and the first resonator grid tovprevent the formation of an ion trap therein.

Fig; 7 shows in longitudinal section view a portion of traps at theresonator gaps as occurred in heretofore employed velocity modulationtubes of this type. a

. Fig. 7(a) isa plot of the potential along the be'anr axis versus thedistance alongthe beam path for the novel beam device shown in Fig. 7and also includesfaf plot of the beam potential. along the-axis of "a.beam. 1 versus" the distance along: the beam path for thehereto- I foreemployedklystrons of .thisgeneral type to more, clearly illustrate theimprovement brought about bythel,

' inventiondisclosed in Fig.7. 1 .=Referring now to Figs. 1 and Zthereis shown arefiepj ldystron, embodying the present invention, therebeing.

shown onlyasuch parts of aprefiex klys'tron: as are necsaryutogillustratelthe present-invention Itshama spaces de:

and. explanationof the. drawings understoodthat the beam path of theparticular klystron would also be enclosed in a vacuum envelope or bodyand the klystron would include other elements such as, for example, anoutput window. The reflect klystron comprises an annular cathode 1having a concave or recessed emitter surface, the cathode surfacesloping radially inwardly and downwardlyas viewed in Fig. I to formsubstantially an acute angle with respect to the axis of the klystrontube. An annular heater 2 is shown positioned on the under side of thecathode to provide the necessary heat for electron emission. An annular,substantially cup-shaped focusing ring 3 encircles the annular cathodeon both its'outer and inner peripheries to provide for focusing of theelectron beam emitted from the cathode. Located within the center orframe opening in the annular focusing ring and cathode and in axialalignment therewith and also extending slightly above the cathodeemitter surface is a metallic collector electrode 4. Positioned in axialalignment with the cathode focusing ring 3 and collector 4 is a cavityresonator 5 including a pair of mutually spaced resonator grids 6 and 7.The drift space or re-entrant tube formed by the outer wall 8 of there-entrant portion of the cavity resonator is of substantially truncatedcone configuration, the sides of which have a slightly convex shape,this particular configuration being employed for a purpose to besubsequently described. Axially aligned with the grids 6 and 7 andpositioned above grid 7 is a metallic concave reflector 9.

In operation, an annular shaped beam of electrons is emitted from thecathode 1 and is focused into a beam by the focusing ring 3, the streamof electrons being accelerated toward the resonator grids 6 and 7 by thepositive potential of the cavity with respect to the cathode as providedby the source of potential represented by battery 11. Because of theangle of the emitter surface of the cathode 1 with respect to the axisthrough the tube, the annular electron beam is directed in such a manneras to tend to form a hollow substantially conical beam, the apex ofwhich coincides with the axis of the klystron and is located within there-entrant tube portion 8, the sides of the beam being somewhat concaveand conforming to the convex surface of the tube re-entrant portion 8.However, the electrons in the annular beam, as they approach the axis,repel each other due to space charge and this interaction between theelectrons tends another trip through the tube and thus the problem ofmultiple transit is eliminated. A recess or pocket 13 is shown in theupper surface of the collector 4 and serves the purpose of retaining anysecondary emission electrons which may be emitted from the collector bythe striking electrons. A decided advantage in the use of the collectorelectrode for catching the electrons after they have performed theiruseful function in the tube is that the electrons are collected on aseparate electrode, the collector, rather than on the walls of the tubeand the cavity resonator, and thus the heat generated by the strikingelectrons does not cause expansion of the cavity resonator or othercritical tube parts with a resultant change in the operating frequencyof the tube.

The collector is connected to a source of potential slightly negativewith respect to the potential of the cavity resonator by the variableresistor 12. The collector, be cause of its slight negative potential,attracts the positive ions which are produced in the re-entrant tube ordrift space 8 of this klystron due to the collision of the electronswith gas molecules in the space. The collector thus continually drainsthe ions from the space and prevents the formation in the beam path of adeleterious ion trap. The particular configuration of the re-entranttube walls 8 also serves to prevent the formation of an ion trap as willbe more readily understood from the following description of this novelfeature disclosed in Fig. 4.

In heretofore existing reflex klystrons wherein attempts were made toprevent the beam from returning into the cathode region after passingthrough the resonator gap, one example of which employs a spike in thecenter of V the reflector to produce an umbrella-shaped reflected beam,the electron beam does not travel parallel to the electric field vectorsacross the resonator gap during both passages thereacross and,therefore, the optimum interchange of energy between the beam andresonator field does not occur. In this present invention, the electronto bend the annular beam within the re-entrant tube portion was to formthe beam into a straight substantially hollow cylindrical beam. Theelectrons in this hollow cylindrical beam configuration pass through thecavity resonator gap between the grids 6 and 7 with the electronstraveling in paths perpendicular to the grids or, as stated in anotherway, parallel to the electric field vectors across the resonator gap.The electrons are velocity modulated by theradio frequency voltagesacross the gap in a well known manner and are repelled by the reflector9 which may carry a negative potential with respect to the cathode. Theelectrons are turned about and again pass through the resonator gap inbunches parallel to the electric field vector across the gap to give upenergy to the field in the cavity resonator. The stream of electronscontinues axially through the reentrant tube 8 and are collected on thecollector 4 which is connected to a source of potential positive withrespect to the cathode, the potential of which may be made variable asrepresented by the variable resistor 12. As noted, the returningelectrons are in the form of a cylindrical beam and, since the diameterof this beam is determined in part by the diameter of the resonatorgrids, the collector electrode upper surface is shown having a diameterequal to or slightly larger than the diameter of the grids. Since thereturned electrons are all collected on the positive collectorelectrode. 4, few, if any, will, re-enter the. region of the negativelycharged oath ode Lwhere. they would beturned about and started .on

beam travels parallel to the electric field vectors across the resonatorgap on both passages therethrough to give the maximum energy exchangeeven though the electron beam does not re-enter the negative cathoderegion on its return trip through the klystron.

Referring to Figure 3 there is shown a portion of a reflex klystron tubeof the heretofore employed type including the cathode 16, the focusingring 17, the walls 18 forming the cylindrical re-entrant or drift spaceand the first resonator grid 19. The outer periphery surface of the beamemitted from the cathode and focused by the focusing ring is shown indotted lines. It is noted that at the entrance to the re-entrant tubespace the beam occupies substantially the entire opening, the beamoccupying a progressively smaller portion of the cylindrical drift tubespace as it proceeds toward the resonator grid 19. The beam may spreadagain as it approaches the grid 19 but, in any case, the beam occupies asmaller proportion of the drift space at some point within the spacedefined by walls 18 than at the left-hand end of the drift space. Thisparticular relationship between the walls of the drift tube space andthe beam space produces an ion trap within the drift tube. The amount ofpotential depression within a drift tube due to the passage of anelectron beam through the drift tube depends on the beam voltage, on thebeam current, on the distribution of current across the beam, and on thegeometry of the drift'tube. In particular, increasing the diameter of acylindrical drift tube, keeping all other quantities constant, resultsin an increased potential depression.

This ion trap is better illustrated in Fig. 3(a) which is a plot of thepotential along the beam axis between the entrance to the re-entranttube 18 and the resonator grid 19. .The ion trap is formed where thepotential curve drops below the positive value which is present attheleft-hand end of the re-entrant tube or drift space due to the spacecharge eflects of the electronbeam.

This ion trap is represented by the cross-hatched area 21. The positiveions in excessnof the number necessary to fill the trap'or', in otherwords, to balance .thedecreas'e in positive potential of the beam willdrain out the lefthand end of the re-entrant tube. The present inventorshave devised a 'novel drift tube structure whichprevents the formationof ion traps, this novel structure employed in Fig. l is being shown inFig. 4.

Referring to this Fig. 4 and also to Fig. 4(a) it :will benoticed thatthe drift tube space, rather' than being of a cylindrical shape, is nowof an approximately truncated cone shape with convex walls, the wallstapering towardthe right-hand or grid end; The cathode 22 and focusingring 23 maybe of the same construction as shown in Fig. 3 and the beamproduced thereby of the same shape as that in Fig. 3. The drift tubewalls are so shaped with relation to the beam perimeter that as the beamprogresses toward the resonator grid 24 it occupies a progressivelyincreasing portion of' the drift tube space. This particularrelationship between the drift tube walls and the 'elect'r'oiibeamprevents the formation in the beam path of any ion traps. This is betterillustrated in Fig. 4(a) which shows a plot of the potential along thebeam axis between the left-hand end of there-entrant tube and theresonator grid. As

can be seen there is no depression or decrease in the positive potentialalong this drift space length so that substantially all of the positiveions formed in the drift tube space drain out the left-hand end thereof.

It is this particular feature of the re-entrant tube portion of theklystron in Fig. l which aids in preventing the formation of ion trapsin the re-entrant tube space of this tube as mentioned above.

Referring to Fig 5 there is shewn therein in section view a portion of areflex klystron tube of the type shown in Fig. 1, there being disclosedonly the reflector electrode 26, the two resonator grids 27 and 28 and aportion of the walls 29 defining the re-entrant tube and cavityresonator. It will be noted that the second resonator grid 27 is thinnerthan the first resonator grid 28 and also that the openings orinterstices defined by the vanes of the grid 27 are substantially largerthan those of the first resonator grid. This particular type of secondrescnator grid permits a portion of the negative field produced by thereflector electrode 2.6 to penetrate through this grid into theresonator gap space between the two grid, thus providing for thedraining of positive ions from the resonator gap to the reflector andthereby preventing the formation in the resonator gap of an ion trap.

In Fig. 6 is disclosed still another novel structure for use in electronbeam apparatus for draining ions from drift spaces. There is shown justthat much of a two-cavity resonator klystron which suffices to explainthe present invention, it being understood that the apparatus shown, orat least the beam path, is enclosed within a vacuum type envelope andmay include other structural features not shown. An annular cathode 31having a slightly concave emitter surface produces an annular beam ofelectrons when heated by a heater 32 extending under the cathode. Thisstream of electrons is focused by a circular focusing member 33 having asubstantially -shaped cross section, the upwardly extending cylindricalcentral portion 34 of this focusing member extending within the centralopening of the annular cathode 31 and in axial alignment with theklystron. An annular accelerating grid 35 is positioned in the path ofthe electron beam for accelerating the electrons therein to a constantvelocity within the space formed by walls 36 of the tube. The beampasses through the gap formed 7 by the resonator grids 37. and 38 of thefirst or buncher cavity resonator 39 where radio frequency energy actson the beam for velocity modulating the electrons therein, the electronsthen passing through the drift space 64 where they become densitymodulated and then passing across the second resonator gap formed by theoutput resonator grids 141 and .42.where .the bunchedelectrons give upenergyto the output cavityresonator 43. The electrons in the beam thencollect on the collector electrode 44. Since in customary operation ofbeam discharge .tubes of .this type the focus electrode 33 is at anegative potential with respect to the walls .of-the drift tube spaceandthe cavity resonators and since in this particular embodiment thecentral upwardly extending portion 34 of the focusing electrode 33extends slightly above the cathode 31 and is aligned with the centralopening in the annular accelerating grid 35, .ions will be drained outfrom the drift ftubespace'by. the negative 7 potential on the focuselectrode through the central opening in the annular accelerator grid tothus prevent formation of ion traps in the drift .tube spaceQ Fig. 7discloses another embodiment of the present inventionwherein the iondraining feature disclosed in Fig. 4 is applied to a multicavitygridless klystron tube which maybe, for example, of the .high poweramplifier class such as disclosed in the US. patent application, SerialNo. 370,568, of Wayne G. Abraham and Sigurd F. Varian entitled HighFrequency Tube filed on July 27, 1953, now US. Patent No. 2,879,440. Theelectron beam emitted from the cathode 51 is transmitted down the seriesof drift tubes 52, 53, 5.4 and 55, the ends of which define resonatorgaps in the three cavity resonators 56,

57 and 58. The inner diameter of each succeeding drift tube along thebeam path is slightly smaller than the inner diameter of the precedingdrift tube, the drift tube inner diameters of each drift tube beingconstant. The electrons in the beam are expended in the collector end ofthe tube. In heretofore employed velocity modulation tubes of this typethe drift tubes had equal and constant inner diameters throughout thelength of the tube as shown in the above cited patent application. Inthese heretofore employed multicavity resonator tubes a potentialdepression is produced at each of the resonator gaps, these depressionsproducing ion traps at each gap. This is better illustrated in Fig. 7(a)which is a plot of the potential along the beam axis versus the distancealong the beam path. The potential curve labeled 61 is typical of theheretofore employed multicavity klystrons while the curve 62 which isvertically transposed on the graph with respect to curve 61 is that ofthe novel tube structure shown in Fig. 7. As seenin curve 61, thepotential depressions at the resonator gaps produce ion traps 63 whichare designated by the cross-hatched area. As indicated by the curve 62representing the novel struc ture shown in Fig. 7, these ions traps areeliminated due to the fact that as the curve is traced from left toright there occur no potential depressions where ion traps may beformed. In addition to preventing the formation of' ion traps at theresonator gaps, this novel configuration also permits the rapid drainingof ions formed throughout the entire length of the beam path due to thefact that the beam potential curve 62 decreases in its positive valuefrom right to left, whereas the beam potential curve 61 is a fiat orunchanging curve. This illustrates another embodiment where an increasedportion of the drift space is occupied by the electron beam as it passesthrough the tube.

Although the klystron disclose in Fig. 7 has each succeeding drift tubeof a smaller inner diameter than the preceding drift tube, the greatestbenefit in tube operation is derived from the elimination of the iontrap at the first resonator gap and a lesser benefit by eliminating theion trap at the second resonator gap. This is due to the fact that anyambiguities in operation that appear early in the stage of thisamplifier tube are amplified to a great extent as the beam proceedsdown, the tube. If such a high degree of perfection in operation is notnecessary, the drift tube 55 may have the same sized inner diameter asthe drift tube 54 or, to carry it a step further,

7 the inner diameters of drift tubes 53, 54 and 55 may be of the samevalue or drift tubes 54 and 55 may be made with larger inner diametersthan drift tube 53 in certain cases, for example, to accommodatespreading of the beam. In these latter examples, ion traps would occurat the resonator gaps in resonators 57 and 58 but not at the resonatorgap in resonator 56. These latter examples were given to illustrate theflexibility of this particular embodiment of the invention.

As an example of the application of this invention, the inner diametersof the drift tubes of a power amplifier tube of the type disclosed inthe above cited patent are each 1.2". To achieve the improvementdescribed herein this power amplifier tube would be modified such as,for example, changing the inner diameter of the first drift tube to1.5", the inner diameter of the second drift tube to 1.3" and the innerdiameter of the third and fourth drift tubes to 1.1".

Since many changes could be made in the above construction of the novelinvention and many apparently widely diflerent embodiments of thisinvention could be made without departing from the scope thereof as, forexample, its utilization in other types of klystrons, in traveling wavetubes and in other electron beam devices, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

This application is a division of my application Ser. No. 348,018 filedOctober 5, 1953, and entitled Electron Discharge Apparatus, now PatentNo. 2,888,599.

What is claimed is:

A reflex klystron including a cavity resonator having a pair ofresonator grids therein, each grid having a plurality ofelectron-permeable interstices, and a reflector aligned with theresonator grids wherein the resonator grid nearest to the reflector issubstantially thinner than the other resonator grid and wherein theinterstices of the first mentioned grid are substantially larger thanthe interstices in said other grid, to thereby cause ions formed in thegap between the resonators to be drained through the thinner,larger-intersticed grid to the reflector.

References Cited in the file of this patent UNITED STATES PATENTS2,518,954 Steele Aug. 15, 1950 2,567,674 Linder Sept. 11, 1951 2,645,739Fremlin et a1. July 14, 1953 2,758,245 Varian Aug. 7, 1956

